1. Field of the Invention
The present invention relates to a low inertial variable speed induction motor. More particularly, it relates to a low inertial variable speed induction motor imparting a relatively low revolving energy to a torque.
2. Description of the Prior Art
Usually, steep rising and falling characteristics of a rotating speed have been required for a servomotor for numerical control or a variable speed motor for an industrial sewing machine. For example, the steep rising to a predetermining sewing speed is required at the starting sewing operation of an industrial sewing machine and the steep falling to a predetermined stopping speed is required at the stopping operation.
It has been considered that a DC motor driven by Thyristor-Leonard control electric power is suitable as the motor used for this purpose, because the DC motor has excellent controllability, a large starting torque and excellent speed response. However, a permanent magnet, a commutator, and brush holders are needed for the DC motor. Moreover, it is difficult to impart high revolving speed in view of the rectifying characteristic. The compact and light weight functions are limited and maintenance in the wearing of brushes is required.
On the contrary, an induction motor can be used without special maintenance. If an economical inverter-controlled power source is obtained, the DC motors may be substantially replaced by the induction motors. However, in the conventional variable speed induction motor, a rotor is fixed to a yoke for forming a revolving field by the mutual functions of the stator and the yoke. A driving body is formed by the large heavy parts of the yoke and the rotor. Moreover, the motor has no means for braking the driving body. Therefore, the conventional variable speed induction motor has high revolving energy to the torque of the driving body, and the rotation of the driving body can not be forcibly braked. Therefore, it can not be used as a motor having steep rising or falling response. In order to use the conventional induction motor for the aforementioned purpose, it is disadvantageously necessary to drive the load through a special device such as an electromagnetic junction in a sewing machine, etc.